Bottom entry plastic ball valve

ABSTRACT

A bottom entry plastic ball valve comprises a valve stem cap ( 1 ), a valve stem ( 2 ), a valve stem sealing ring ( 3 ), a valve stem end surface sealing ring ( 4 ), a valve ball sealing ring ( 5 ), a supporting ring ( 6 ) for the valve ball sealing ring, a valve ball ( 7 ), a connecting pipe ( 8 ), and a valve body ( 9 ). A hollow valve ball chamber ( 911 ) is provided inside the valve body ( 9 ), the valve body ( 9 ) is provided with a valve bottom cover hole ( 912 ) just below the valve ball chamber ( 911 ), and the width of the valve bottom cover hole ( 912 ) is larger than the diameter of the valve ball ( 7 ). The ball valve also comprises a valve bottom cover ( 10 ) corresponding to the valve bottom cover hole ( 912 ) so that the valve bottom cover ( 10 ) can cover the valve bottom cover hole ( 912 ) and can be fusion-bonded to the valve body ( 9 ), thereby avoiding the need for electric fusion welding in the manufacture of large-diameter ball valves.

The present application is the national phase of InternationalApplication No. PCT/CN2009/073420, titled “BOTTOM ENTRY PLASTIC BALLVALVE”, filed on Aug. 21, 2009, which claims the benefit of priority toChinese patent application No. 200910040476.7 titled “BOTTOM ENTRYPLASTIC BALL VALVE”, filed on Jun. 23, 2009. The entire disclosurethereof is incorporated herein by reference.

FIELD OF THE INVENTION

The invention belongs to the field of plastic ball valves, andparticularly relates to a bottom entry plastic ball valve.

BACKGROUND OF THE INVENTION

With wide application of plastic gas pipelines, plastic ball valve isfirstly developed and manufactured in USA and widely applied to plasticgas pipeline systems to meet the needs of forming pipeline systems.People have been gradually realized the superiority of plastic ballvalves in respects of performances/price and began to replace metal ballvalves with plastic ball valves.

For traditional plastic valves, a small-diameter ball valve isintegrated by hot melt welding, and a large-diameter ball valve adoptselectric fusion welding. The hot melt welding has simple process, lowcost and relative low accuracy; and the electric fusion welding hascomplex process and high cost.

Presently, the valve structure with hot melt welding commonly adopts anintegral side-entry structure, in which a ball body, a valve stem and asealing ring of one side are assembled in a valve body form one end ofthe valve body, the diameter of the end is equal to or larger than thediameter of a valve ball chamber; the other sealing ring is mounted on aconnecting pipe seat, the valve body and the connecting pipe seat arewelded together to form an integral ball valve by hot melt welding orelectric fusion welding between the inner cylindrical surface withlarger diameter where the valve body assembled in the ball body and theouter cylindrical surface of the connecting pipe seat. The valve stem isbuckled with a valve stem cap through the buckling part of the valvestem, and the valve stem cap is buckled with the valve body through thebuckling part of the valve stem cap. The two sealing rings arerespectively fixed on the valve body and the connecting pipe. Inassembly, the valve stem cap is firstly buckled with the valve bodythrough the buckling part, then a sealing ring and a supporting ringthereof are mounted in a sealing ring groove and a supporting ringgroove from the end with larger diameter of the valve body, next, thevalve stem is mounted from the end with larger diameter of the valvebody in a way that the buckling part of the valve stem is buckled in thebuckling part of the valve stem cap, and finally, the valve ball ismounted; the sealing rings are mounted on a sealing ring mounting ringof the valve body and a connecting pipe seat sealing ring, then theinner cylindrical surface of the insertion-accepting part of the valvebody and the outer cylindrical surface of the connecting pipe are heatedand molten on a hot melting machine, and when the two cylindricalsurfaces are molten to a certain thickness, the connecting pipe isinserted into the insertion-accepting part of the valve body to acertain depth and gradually cooled by keeping pressure.

Such structure has the following defects:

I. The bottom of the valve stem has an end surface on which an O-shapedsealing ring is mounted through an annular groove provided on the endsurface, a plane perpendicular to the valve stem is milled on the valvebody correspondingly, the buckling part of the valve stem and thebuckling part of the valve stem cap are required to cause the valve stemto maintain a proper force pointing at the outside of the valve body,such that the O-shaped sealing ring on the end surface of the valve stemcan seal the end surface of the valve stem by a proper pressing force.However, in such structure, the designed pressing force is obtainedthrough the two buckling parts on three parts, which brings moredifficulties for manufacture and assembly;

II. The insertion depth of the connecting pipe in the valve bodydirectly affects the axial distance between two sealing surfaces andinfluences a pre-stressed force. Large insertion depth causes largepre-stressed force and large torques for opening and closing the valveand serious abrasion; small insertion depth brings small pre-stressedforce of the sealing surface, thereby causing a hidden danger of innerleakage. In addition, it is difficult to ensure coincidence of centrallines of the valve body and the connecting pipe during insertion, thetwo central lines often do not coincide and form a certain angle,resulting in uneven pressing force between the sealing rings and theball body on a whole sealing area;

III. From the view of manufacture, because the two sealing rings arerespectively positioned on the valve body and the connecting pipe, andprocess positioning surfaces for machining mounting rings of the sealingrings are respectively positioned on two mounting rings, the axialdistance between the mounting rings of the sealing rings after assemblyhas an error due to the manufacture of the positioning surfaces.

SUMMARY OF THE INVENTION

The object of the invention is to provide an integral bottom entry valvebody structure to overcome the defects of the prior art.

In order to realize the above object, the invention employs thefollowing technical scheme:

A bottom entry plastic ball valve comprises a valve stem cap, a valvestem, a valve stem sealing ring, a valve stem end face sealing ring, avalve ball sealing ring, a supporting ring for the valve ball sealingring, a valve ball, a connecting pipe and a valve body. A hollow valveball chamber is provided inside the valve body, the valve body isprovided with a valve bottom cover hole just below the valve ballchamber, the width of the valve bottom cover hole is larger than thediameter of the valve ball, the ball valve also comprises a valve bottomcover corresponding to the valve bottom cover hole so that the valvebottom cover can cover the valve bottom cover hole and can befusion-bonded to the valve body.

The plastic ball valve of the invention is characterized in that thevalve bottom cover hole is provided just below the valve ball chamberfor mounting two valve ball sealing rings and a valve ball, and thevalve bottom cover covers the valve bottom cover hole and adopts endsurface welding through hot melt, thereby avoiding the need for electricfusion welding in manufacture of large-diameter valves.

In the above technical scheme, said valve bottom cover hole is parallelto the axial line of the valve body, and the valve bottom cover hole isin the shape of a square, or a rectangle, or a circle, or an ellipse.

An annular plane is provided around the valve bottom cover hole to betaken as a valve bottom cover melt adhesive surface which is parallel tothe axial line of the valve body, and the valve body and the valvebottom cover are in end surface welding through the valve bottom covermelt adhesive surface by hot melt.

As another structure, a conical surface is provided around the valvebottom cover hole to be taken as a valve bottom cover melt adhesivesurface which is perpendicular to the axial line of the valve body, andthe valve body and the valve bottom cover are in end surface weldingthrough the valve bottom cover melt adhesive surface by hot melt.

The right end and the left end of the valve body are respectivelyprovided with a connecting pipe seat, the side of said each connectingpipe seat fusion-welded with the connecting pipe is provided with anouter end surface ring which is perpendicular to the axial line of thevalve body, and the valve body and the connecting pipe are in endsurface welding through the outer end surface rings by hot melt. Theouter end surface rings are perpendicular to the axial line of the valvebody, and because the outer end surface rings are close to the valveball chamber, the sealing ring groove and the sealing ring supportingring groove can be turned conveniently by a numerical control lathe; andanother important function of the outer end surface rings lies informing end surface welding with the connecting pipe by hot melt,avoiding the need for electric fusion welding process in manufacture oflarge-diameter ball valves.

The connecting pipe is provided with a fusion welding surface used forbeing welded with the valve body by hot melt, and said fusion weldingsurface is of an annular end surface or a conical surface andcorresponds to the outer end surface ring which is an annular plane or aconical surface.

Both sides of the valve ball chamber of the valve body are respectivelyprovided with a positioning end surface for the mounting ring for thevalve ball sealing ring and a positioning end surface for the supportingring for the valve ball sealing ring. The mounting ring positioning endsurface for the valve ball sealing ring and the supporting ringpositioning end surface for the valve ball sealing ring are machined onboth sides of the valve ball chamber of the valve body, that is to saythe two valve body sealing rings and the two supporting rings for thevalve ball sealing rings are mounted on the valve body, which isdifferent from two valve sealing rings of the traditional ball valve inmounting mode and position, and as for traditional ball valve sealingrings, one is mounted in from one end of the valve body, and the otheris mounted on the connecting pipe.

Said valve ball chamber is an inner cylindrical surface coaxial with thevalve body and has an indentation at the valve bottom cover hole, saidvalve bottom cover is provided with an inner cylindrical surface of thevalve ball chamber for supplementing the indentation, the valve body isfurther provided with an inner cylindrical surface fitted with the outercylindrical surface of the valve ball sealing ring, the innercylindrical surface is in the shape of a circular ring concentric withthe valve body, the inner cylindrical surface has an indentation on thevalve body, and the valve bottom cover is provided with an innercylindrical surface of the valve ball sealing ring for supplementing theindentation.

Said valve bottom cover comprises an inner cylindrical surface of thevalve ball chamber, an inner cylindrical surface of the valve ballsealing ring, a fusion welding end surface, an axial positioning endsurface of the valve bottom cover and a radial positioning cylindricalsurface of the valve bottom cover, wherein the inner cylindrical surfaceof the valve ball chamber and the inner cylindrical surface of the valveball chamber form an integral inner cylindrical surface of the valveball; the inner cylindrical surface of the valve ball sealing ring andthe inner cylindrical surface of the sealing ring form an integralradial positioning inner cylindrical surface for the valve ball sealingring; the fusion welding end surface is welded with the melt adhesivesurface of the valve bottom cover through hot melt welding and has thesame profile with the melt adhesive surface of the valve bottom cover;the axial positioning end surface of the valve bottom cover is parallelto the fusion welding end surface and fitted with the axial positioningsurface of valve bottom cover on the valve body, has the same shape withthe axial positioning surface of the valve bottom cover and limits thevalve bottom cover in axial direction; and the radial positioningcylindrical surface of the valve bottom cover is fitted with acircumferential positioning surface on the valve body and limits thevalve bottom cover in radial direction.

The invention has the following assembly process:

An axial sealing ring of the valve stem and a valve stem end surfacesealing ring are firstly mounted in corresponding sealing ring mountinggrooves on the valve body through the valve bottom cover hole, and thenthe valve stem is mounted in so that the valve stem is buckled with anaxial limiting buckling part on the valve body; two valve ball sealingrings are mounted in corresponding sealing ring mounting grooves on thevalve body through the valve bottom cover hole, then a ball body ismounted in the valve ball chamber through the valve bottom cover hole,and next, two supporting rings for the valve ball sealing rings arerespectively pressed in the valve body from the right side and the leftside of the valve body; the valve bottom cover, two connecting pipes andthe valve body are fusion-welded together through hot melt; and finally,the valve stem cap is assembled.

In the invention, the ball body adopts a bottom entry structure, whereintwo valve ball sealing rings are directly mounted on the valve body suchthat the axial distance between the two sealing rings can be accuratelycontrolled, and accordingly, the pre-stressed force of the sealingsurface and the torsional moment of the valve stem can be effectivelycontrolled, thereby solving the problem in the prior art of lowqualification rate because of inner leakage and overlarge torsionalmoment caused by inefficient control of the axial distance betweensealing rings. In addition, because end surface hot melt welding orconical surface hot melt welding is used between the valve bottom coverand the valve body and between the connecting pipes and the valve body,there is no need to use electric fusion welding which has high cost andcomplex fusion welding process in manufacture of large-diameter valves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the whole structure of the PE ball valve of theinvention;

FIG. 2 is a structural diagram of the valve body of the PE ball valve ofthe invention;

FIG. 3 is another structural diagram of the valve body of the PE ballvalve of the invention;

FIG. 4 is a structural diagram of the valve stem of the PE ball valve ofthe invention;

FIG. 5 is a structural diagram of the supporting ring for the sealingring of the PE ball valve of the invention;

FIG. 6 is a structural diagram of the valve bottom cover of the PE ballvalve of the invention;

FIG. 7 is a structural diagram of the valve stem cap of the PE ballvalve of the invention; and

FIG. 8 is a structural diagram of the connecting pipe of the PE ballvalve of the invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is further described by combining the drawings:

Referring to the diagram of the whole structure of the PE ball valve asshown in FIG. 1, the plastic ball valve of the invention comprise avalve stem cap 1, a valve stem 2, a valve stem sealing ring 3, a valvestem end surface sealing ring 4, a valve ball sealing ring 5, asupporting ring 6 for the valve ball sealing ring, a valve ball 7, aconnecting pipe 8, a valve body 9 and a valve bottom cover 10.

Referring to the structural diagrams of the valve body of the PE ballvalve as shown in FIG. 2 and FIG. 3, said valve body 9 substantially isa cylinder which is internally provided with a series of cylindricalholes communicated with each other, and the axial lines of thecylindrical holes coincide with the cylinder body of the valve body inaxial direction. The valve body 9 comprises a melt adhesive surface 91of the valve bottom cover, a circumferential positioning surface 94 ofthe valve bottom cover, an axial positioning surface 95 of the valvebottom cover, an inner cylindrical surface 92 fitting with the valveball sealing ring 5, a mounting ring groove 93 for the valve ballsealing ring, an axial positioning circumferential end surface 96 of thesupporting ring for the valve ball sealing ring, an axial positioningcircumferential end surface 97 of the connecting pipe, a radialpositioning surface 98 of the connecting pipe, a melt adhesivecontaining groove 99, a melt adhesive end surface 910 used for beingfusion-welded with the connecting pipe 8, a valve ball chamber 911, avalve bottom cover hole 912, a circumferential end surface 943 of thevalve stem, a mounting groove 946 for the valve stem end surface sealingring, an axial positioning buckling part groove 945 of the valve stem, alimiting convex ring 949 of the valve stem cap, a limiting cylindricalsurface 9412 of the valve stem cap, a valve stem shaft hole 9410 and anaxial sealing ring mounting groove 944 of the valve stem.

The valve ball chamber 911 of the valve body 9 is an inner cylindricalsurface concentric with the valve body, the inner cylindrical surfacehas an indentation at the valve bottom cover hole 912, the indentationis positioned on the valve bottom cover 10 and combines with an innercylindrical surface 101 of the valve ball chamber to form an integralinside of the valve ball. Said valve bottom cover hole 912 is in theshape of a circle, or a square, or a rectangle and communicated with thevalve ball chamber 911.

On the valve body 9, the inner cylindrical surface 92 fitted with theouter cylindrical surface of the valve ball sealing ring 5 is in theshape of a circular ring concentric with the valve body. The innercylindrical surface 92 has an indentation on the valve body 9, theindentation is positioned on the valve bottom cover 10 and combines withthe inner cylindrical surface 102 of the valve sealing ring of the valvebottom cover 10 to form an integral radial positioning inner cylindricalring of the valve ball sealing ring.

The circumferential end surface 943 of the valve stem is parallel to theaxial line of the valve body 9 and perpendicular to the valve stem shafthole 9410, and is provided with the mounting groove 946 for mounting thevalve stem end surface sealing ring 4, and the axial line of themounting groove 946 coincides with the axial line of the valve stemshaft hole 9410.

Said axial positioning buckling part groove 945 of the valve stem is anannular groove in the shape of a square, or a rectangle, or a triangle,and the axial line thereof coincides with the axial line of the valvestem shaft hole 9410.

Said limiting cylindrical surface 9412 of the valve stem cap isconcentric with the valve stem shaft hole 9410 and is provided with anannular protrusion, i.e. the limiting convex ring 949 of the valve stemcap, with the cross section in the shape of a circle, or a rectangle, oran ellipse, or a rhombus at the upper part. Said limiting convex ring949 of the valve stem cap is fitted with a buckling groove 15 of thevalve stem cap 1 to limit the valve stem cap 1.

Said melt adhesive surface 91 of the valve bottom cover is a flatsurface or a conical surface with rectangular, or square, or circular orelliptical inner and outer profiles. If being a flat surface, the meltadhesive surface 91 is parallel to the axial line of the valve body 9;if being a conical surface, the axial line of the melt adhesive surface91 is perpendicular to the axial line of the valve body 9 and coincideswith the normal line of the outer cylindrical surface of the valve body.The melt adhesive surface 91 is used for enabling the valve bottom cover10 to be welded with the valve body 9 by being welded with the fusionwelding end surface 107 on the valve bottom cover 10 through hot melt.

The mounting ring 93 for the valve ball sealing ring of the valve body 9is a circular ring with a square cross section and is used forpositioning the valve sealing ring 5 in axial direction. The axialpositioning circumferential end surface 96 of the supporting ring forthe sealing ring is a planar ring perpendicular to the axial line of thevalve body and is used for positioning the supporting ring 6 for thesealing ring of the valve body 9 in axial direction.

The axial positioning circumferential end surface 97 of the connectingpipe of the valve body 9 is an annular plane perpendicular to the axialline of the valve body and is used for positioning the connecting pipe 8in axial direction. The radial positioning surface 98 of the connectingpipe of the valve body 9 is an inner cylindrical surface concentric withthe valve body and is used for positioning the connecting pipe 8 inradial direction.

The axial positioning surface 95 of the valve bottom cover is an annularplane with the central line coinciding with the central line of thecircumferential positioning surface 94 and is used for being fitted withthe axial positioning end surface 108 of the valve bottom cover 10 tolimit the valve bottom cover 10 in axial direction.

The mounting groove 946 for the valve stem end surface sealing ring isan annular groove which is positioned on the circumferential end surface943 of the valve stem; the axial positioning buckling part groove 945 ofthe valve stem is positioned on the valve stem shaft hole 9410 andbuckled with an axial positioning buckling part 24 of the valve stem 2to limit the axial movement of the valve stem 2; the axial sealing ringmounting groove 944 of the valve stem is positioned on the valve stemshaft hole 9410; the axial lines of the axial positioning buckling partgroove 945 of the valve stem, the valve stem shaft hole 9410, thelimiting convex ring 949 of the valve stem cap, the limiting cylindricalsurface 9412 of the valve stem cap and the axial sealing ring mountinggroove 944 of the valve stem coincide with each other, are perpendicularto the axial line of the valve body 1 and coincide with the axial lineof the valve bottom cover 10.

Refereeing to the structural diagram of the valve stem as shown in FIG.4, the valve stem 2 comprises a valve ball poke block 21, a valve stemend surface sealing ring 22, an end surface sealing plane 23, an axialpositioning buckling part 24 of the valve stem, a cylindrical section 25of the valve stem and a torsional section 26 of the valve stem. Saidvalve stem end surface sealing ring 22 is a cylindrical ring concentricwith the valve stem 2, and the valve stem end surface sealing plane 23is positioned on the valve stem end surface sealing ring 22 andperpendicular to the axial line of the valve stem 2; said axialpositioning buckling part 24 of the valve stem is positioned on andconcentric with the cylindrical section 25 of the valve stem andprotrudes from the cylindrical section 25 of the valve stem, the axialpositioning buckling part 24 of the valve stem has a conical crosssection which is concentric with the cylindrical section 25, and thebiggest bottom surface of the axial positioning buckling part 24 is aplane closing to the valve stem end surface sealing ring 22 andperpendicular to the axial line of the valve stem. The axial positioningbuckling part 24 of the valve stem is fitted with the axial positioningbuckling part groove 945 of the valve stem on the valve body 9 to limitthe distance of the valve stem 2 towards the valve ball. The torsionalsection 26 of the valve stem 2 are two parallel planes cut on the asegment of a cylinder body concentric with the valve stem 2 and smallerthan the cylindrical section 25 in diameter along the axial line of thevalve stem, and are used for being fitted with a torsional hole in thevalve stem cap 1 and giving a torque to the valve stem to drive thevalve ball to rotate.

Referring to the structural diagram of the supporting ring for the valveball sealing ring as shown in FIG. 5, the supporting ring 6substantially is in the shape of a cylinder, the inner circular surfacecomprises two conical surfaces 61, 67 and a cylindrical surface, and thetwo conical surfaces coincide with the axle center of the cylindricalsurface and coincide with two outer cylindrical surfaces 63, 65. Afterassembly, a conical surface 62 is provided closing to the valve ball.The outer cylindrical surface 63 is fitted with the inner cylindricalsurface of the valve ball sealing ring 5. A circumferential end surface64 is fitted with the circumferential end surface 96 on the valve body 9to position the supporting ring 6 in axial direction. A circumferentialend surface 66 is provided away from the valve ball and fitted with anaxial positioning end surface 83 of the supporting ring of theconnecting pipe 8.

Referring to the structural diagram of the valve bottom cover as shownin FIG. 6, the valve bottom cover 10 mainly comprises an innercylindrical surface 101 of the valve ball chamber, an inner cylindricalsurface 102 of the valve ball sealing ring, a fusion welding end surface107, an axial positioning end surface 108 of the valve bottom cover, anda radial positioning cylindrical surface 1011 of the valve bottom cover.The inner cylindrical surface 101 of the valve ball chamber and theinner cylindrical surface 911 of the valve ball chamber of the valvebody 9 form an integral inner cylindrical surface of the valve ball; andthe inner cylindrical surface 102 of the valve ball sealing ring and theinner cylindrical surface 92 of the sealing ring of the valve body 9form an integral radial positioning inner cylindrical ring of the valveball sealing ring. The axial line of the fusion welding end surface 107coincides with the axial line of the valve stem shaft hole on the valvebody, and is used for being welded with the melt adhesive surface 91 onthe valve body 9 through hot melt process, and has the same profile withthe melt adhesive surface 91 of the valve bottom cover on the valve body9. The axial positioning end surface 108 of the valve bottom cover isparallel to the fusion welding end surface 107, is fitted with the axialpositioning surface 95 of the valve bottom cover on the valve body 9,has the same shape and size with the axial positioning surface 95 of thevalve bottom cover, and plays a role of limiting the valve bottom cover10 in axial direction. The axle center of the radial positioningcylindrical surface 1011 of the valve bottom cover coincides with theaxle center of the valve stem shaft hole and is fitted with thecircumferential positioning surface 94 on the valve body 9 to limit thevalve bottom cover 10 in radial direction.

The structural diagram of the valve stem cap as shown in FIG. 7comprises a sectional axial side view (c) of a quarter of the valve stemcap 1, a sectional view (a) and an enlargement view (b) of part A. Saidvalve stem cap 1 comprises a torsional hexagonal prism 18, a rotarylimiting block 16 of the valve stem cap, a conical surface 11 of a capmouth, a torsional hole 17 fitted with the torsional section 26 of thevalve stem, and an end surface 110 between the rotary limiting block 16and the torsional hole 17. The cross section of the buckling groove 15is in the shape of a square, or an ellipse, or a circle, or a triangle,and the axial line of the buckling groove coincides with the axial linesof the torsional hole 17 and the conical surface 11 of the cap mouth.Said buckling groove 15 closes to the wall surface of the cap mouth andis tangential to the conical surface 11 of the cap mouth at the jointposition through an arc surface 12, a cylindrical surface 13 and an arcsurface 14 to form a smooth curved surface, thereby facilitating thelimiting convex ring 949 of the valve stem cap of the valve body 9 tosmoothly enter the buckling groove 15 and avoiding damage to thelimiting convex ring 949 of the valve stem cap in entry. Said torsionalhole 17 consists of two parallel planes and two opposite cylindricalsurfaces having the same diameter.

Referring to the structural diagram of the connecting pipe as shown inFIG. 8, the connecting pipe 8 is substantially is a revolution body witha round cross section, and comprises a pipe body 85, an axialpositioning end surface 83 of the supporting ring used for limiting thesupporting ring 6 for the valve ball sealing ring in axial direction, aradial limiting cylindrical surface 82, a fusion welding end surface 81and an adhesive containing groove 84. Said limiting end surface 83contacts with the circumferential end surface 66 of the supporting ring6 and limits the supporting ring 6 in axial direction. Said fusionwelding surface 81 is an annular plane perpendicular to the axial lineof the pipe body 85 or a conical surface coinciding with the axial lineof the pipe body 85 and is used for being welded with the melt adhesivesurface 910 of the connecting pipe 8 on the valve body 9 through hotmelt. Said adhesive containing groove is an annular groove with thecross section in the shape of a square, or a rectangle, or a circle, orother shapes and is used for containing excess molten plastic during hotmelt insertion.

The invention claimed is:
 1. A bottom entry plastic ball valvecomprising a valve stem cap (1), a valve stem (2), a valve stem sealingring (3), a valve stem end surface sealing ring (4), a valve ballsealing ring (5), a supporting ring (6) for the valve ball sealing ring,a valve ball (7), a connecting pipe (8), and a valve body (9), wherein ahollow valve ball chamber (911) is provided inside the valve body (9),wherein the valve body (9) is provided with a valve cover hole (912)just below the valve ball chamber (911), the width of the valve coverhole (912) is larger than the diameter of the valve ball (7), the ballvalve also comprises a valve cover (10) corresponding to the valve coverhole (912) so that a valve cover (10) can cover the valve cover hole(912) and can be fusion-bonded to the valve body (9), wherein the rightend and the left end of the valve body (9) are respectively providedwith a connecting pipe seat, the side of said each connecting pipe seatfusion-welded with the connecting pipe (8) is provided with an outer endsurface ring (910) which is perpendicular to the axial line of the valvebody (9), and the valve body (9) and the connecting pipe (8) are in endsurface welding through the outer end surface rings (910) by hot melt,wherein two sides of the valve ball chamber (911) of the valve body (9)are respectively provided with a positioning end surface for themounting ring for the valve ball sealing ring and a positioning endsurface for the supporting ring for the valve ball sealing ring, andwherein the valve ball chamber (911) is an inner cylindrical surfacecoaxial with the valve body (9) and has an indentation at the valvecover hole (912), the valve cover (10) is provided with an innercylindrical surface (101) of the valve ball chamber for supplementingthe indentation, the valve body (9) is further provided with an innercylindrical surface (92) fitted with the outer cylindrical surface ofthe valve ball sealing ring (5), the inner cylindrical surface (92) isin the shape of a circular ring concentric with the valve body (9), theinner cylindrical surface (92) has an indentation on the valve body (9),and the valve cover (10) is provided with an inner cylindrical surface(102) of the valve ball sealing ring for supplementing the indentation.2. The bottom entry plastic ball valve according to claim 1, wherein thevalve cover hole (912) is parallel to the axial line of the valve body(9), and the valve cover hole (912) is in the shape of a square, or arectangle, or a circle, or an ellipse.
 3. The bottom entry plastic ballvalve according to claim 1, wherein an annular plane is provided aroundthe valve cover hole (912) to be taken as a valve cover melt adhesivesurface (91) which is parallel to the axial line of the valve body (9),and the valve body (9) and the valve cover (10) are in end surfacewelding through the valve cover melt adhesive surface (91) by hot melt.4. The bottom entry plastic ball valve according to claim 1, wherein theconnecting pipe (8) is provided with a fusion welding surface (81) usedfor being welded with the valve body (9) by hot melt, and the fusionwelding surface (81) is of an annular end surface or a conical surfaceand corresponds to the outer end surface ring (910) which is an annularplane or a conical surface.
 5. The bottom entry plastic ball valveaccording to claim 1, wherein the valve cover (10) comprises an innercylindrical surface (101) of the valve ball chamber, an innercylindrical surface (102) of the valve ball sealing ring, a fusionwelding end surface (107), an axial positioning end surface (108) of thevalve cover and a radial positioning cylindrical surface (1011) of thevalve cover, wherein the inner cylindrical surface (101) of the valveball chamber and the inner cylindrical surface (911) of the valve ballchamber form an integral inner cylindrical surface of the valve ball;the inner cylindrical surface (102) of the valve ball sealing ring andthe inner cylindrical surface (92) of the sealing ring form an integralradial positioning inner cylindrical surface for the valve ball sealingring; the fusion welding end surface (107) is welded with the meltadhesive surface (91) of the valve cover through hot melt welding andhas the same profile with the melt adhesive surface (91) of the valvecover; the axial positioning end surface (108) of the valve cover isparallel to the fusion welding end surface (107) and fitted with theaxial positioning surface (95) of valve cover on the valve body (9), hasthe same shape with the axial positioning surface (95) of the valvecover and limits the valve cover (10) in axial direction; and the radialpositioning cylindrical surface (1011) of the valve cover is fitted witha circumferential positioning surface (94) on the valve body (9) andlimits the valve cover (10) in radial direction.